U.S. patent number 6,723,119 [Application Number 09/864,160] was granted by the patent office on 2004-04-20 for longitudinally flexible stent.
This patent grant is currently assigned to Medinol Ltd.. Invention is credited to Gregory Pinchasik, Jacob Richter.
United States Patent |
6,723,119 |
Pinchasik , et al. |
April 20, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Longitudinally flexible stent
Abstract
An intravascular stent especially suited for implanting in
curved arterial portions. The stent retains longitudinal
flexibility after expansion. The stent includes a plurality of
first circumferential bands containing a pattern of loops at a
first frequency and a plurality of second circumferential bands
containing a pattern of loops at a second frequency higher than
said first frequency, alternating with said first circumferential
bands and periodically coupled thereto to form cells. The high
frequency elements provide a flexibility after expansions which can
be repeatedly stress by the beating heart, with out exceeding the
elastic limit of the stent material.
Inventors: |
Pinchasik; Gregory (Herzlia,
IL), Richter; Jacob (Ramat Hasharon, IL) |
Assignee: |
Medinol Ltd. (Tel Aviv,
IL)
|
Family
ID: |
25342652 |
Appl.
No.: |
09/864,160 |
Filed: |
May 25, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
795794 |
Feb 28, 2001 |
|
|
|
|
516753 |
Mar 1, 2000 |
|
|
|
|
Current U.S.
Class: |
623/1.15 |
Current CPC
Class: |
A61F
2/91 (20130101); A61F 2/915 (20130101); A61F
2002/91508 (20130101); A61F 2002/91525 (20130101); A61F
2002/91533 (20130101); A61F 2002/91558 (20130101); A61F
2230/0054 (20130101) |
Current International
Class: |
A61F
2/06 (20060101); A61F 002/06 () |
Field of
Search: |
;623/1,1.11,1.12,1.13,1.14,1.15,1.16,1.17,1.18,1.19,1.2,1.21
;608/1.08,191,194,195,198 ;600/192 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
195 12 066 |
|
Nov 1996 |
|
DE |
|
19957063 |
|
Aug 2001 |
|
DE |
|
0 830 853 |
|
Mar 1998 |
|
EP |
|
0 830 853 |
|
Mar 1998 |
|
EP |
|
0 970 664 |
|
Jan 2000 |
|
EP |
|
0876216 |
|
Apr 2000 |
|
EP |
|
280547 |
|
Sep 1998 |
|
NZ |
|
285241 |
|
Mar 1999 |
|
NZ |
|
331532 |
|
Jan 2000 |
|
NZ |
|
WO 96/26689 |
|
Sep 1996 |
|
WO |
|
WO 97/07889 |
|
Mar 1997 |
|
WO |
|
WO 97/32544 |
|
Sep 1997 |
|
WO |
|
WO 98 35634 |
|
Aug 1998 |
|
WO |
|
WO 98/35634 |
|
Aug 1998 |
|
WO |
|
99/39660 |
|
Aug 1999 |
|
WO |
|
WO 99/44543 |
|
Sep 1999 |
|
WO |
|
99/62431 |
|
Dec 1999 |
|
WO |
|
WO 00/30563 |
|
Jun 2000 |
|
WO |
|
Other References
Singapore Examination Report. .
New Zealand Examination Report (May 24, 2001) & (Mar. 6, 2001).
.
German Office Action (1 page, File # 10109508.2-43). .
Cancellation Proceeding against DE Patent No. 20108764. .
Cancellation Proceeding against DE Patent No. 20108765. .
Two European Search Reports. .
PCT Search Reports..
|
Primary Examiner: Bui; Vy Q.
Attorney, Agent or Firm: Morgan & Finnegan, LLP
Parent Case Text
RELATED APPLICATIONS
This application is a continuation-in-part of Ser. No. 09/795,794
filed Feb. 28, 2001, which is a continuation-in-part of Ser. No.
09/516,753 filed Mar. 1, 2000 and which also claims the priority of
Provisional Application No. 60/202,723, filed May 8, 2000.
Claims
What is claimed is:
1. A stent comprising a first loop containing section, the first
loop containing section arranged generally in the circumferential
direction, the loops in said first loop containing section
occurring at a first frequency; a second loop containing section,
the second loop containing section arranged generally in the
circumferential direction the loops in said second loop containing
section also occurring at said first frequency and a third loop
containing section, the loops in said third loop containing section
occurring at a second frequency that is higher than said first
frequency, the third loop containing section disposed in the
generally circumferential space between said first and second loop
containing sections and alternately joined to said first and second
loop containing sections such that said first and second loop
containing sections are joined together through the third loop
containing section without connection directly between the first
and second loop containing sections, wherein the first and second
loop containing sections have two cycles for every three cycles of
said third loop containing section, wherein the first loop
containing section or the second loop containing section and the
third loop containing section form at least one cell, the first
loop containing section or the second loop containing section in
the at least one cell further has at least one loop longitudinally
shorter than another loop in the first loop containing section or
the second loop containing section.
2. A stent according to claim 1, wherein the first loop and second
loop containing sections are relatively adapted to enable radial
support and the third loop containing section is relatively adapted
to enable longitudinal flexibility.
3. A stent according to claim 1, wherein the first loop and second
loop containing sections have wider struts than the third loop
containing section.
4. A stent according to claim 1, wherein the relative widths of
said first, second, and third loop containing sections are such
that when said stent is crimped for insertion into a lumen of a
blood vessel, said third loop containing section is crimpable to
essentially the same diameter as said first loop and second loop
containing sections.
5. A stent according to any of claim 1, wherein the second
frequency elements provide improved flexibility.
6. A stent according to claim 5, wherein, while flexing, the second
frequency elements have maximal strain that is lower than the
elastic limit for the material of the stent.
7. A stent according to claim 6, wherein, the maximal strain of the
expanded stent within a blood vessel caused by repeated flexing is
below the strain which would cause permanent deformation for the
material of the stent.
8. A stent according to claim 7, wherein, said stent is made of
stainless steel and said maximal strain is below approximately
0.5%.
9. A stent according to any of claim 8, wherein the first and
second loop containing sections are 180 degrees out of phase with
each other.
10. A stent according to claim 1, wherein substantially each cell
in the stent encompasses the same area.
11. A stent according to claim 1, wherein at least one cell is
arranged so that when expanded a length of the cell along a
circumference of the stent is longer than a length of a cell along
the longitudinal axis of the stent.
12. A stent according to claim 1, wherein at least one cell of the
stent is symmetrical about a line parallel to a longitudinal axis
of the stent.
13. A stent comprising: a plurality of first circumferential bands
containing a pattern of loops at a first frequency, a plurality of
second circumferential bands containing a pattern of loops at a
second frequency higher than said first frequency, alternating with
said first circumferential bands and periodically coupled thereto
to form cells such that said first circumferential bands are joined
together through said second circumferential bands without
connection directly between said first circumferential bands
wherein the first circumferential bands containing a pattern of
loops are comprised of even first circumferential bands containing
a pattern of loops and odd first circumferential bands containing a
pattern of loops which are 180 degrees out of phase with the loops
of the even first circumferential bands, an odd first
circumferential band occurring between every two even first
circumferential bands, wherein each cell includes two cycles of one
of said plurality of first circumferential bands and three cycles
of one of said plurality of second circumferential bands such that
the first circumferential band has at least one loop shorter than
another loop in the first circumferential band.
14. A stent according to claim 13, wherein each cell includes a
number of loops of said first circumferential band corresponding to
two cycles of said first frequency and a number of loops of said
second circumferential band corresponding to three cycles of said
second frequency.
15. A stent according to claim 13, wherein the first
circumferential bands have struts that are wider than the struts in
said second circumferential bands.
16. A stent according to claim 15, wherein the relative widths of
said loops is such that when said stent is crimped for insertion
into a lumen of a blood vessel, the loops of said second
circumferential bands are crimpable to essentially the same
diameter as the loops of said first circumferential bands.
17. A stent according to claim 15, wherein the higher frequency of
the loops in said second circumferential bands provide improved
flexibility.
18. A stent according to claim 17, wherein, while flexing, elements
in the higher frequency loops have lower maximal strain.
19. A stent according to claim 18, wherein, the maxima strain of
the expanded strain within a blood vessel cause by repeated flexing
is below the maximum strain causing permanent deformation for the
material of the stent.
20. A stent according to claim 18, wherein said lower maximal
strain is below approximately 0.5%.
21. A stent according to claim 13, wherein the first
circumferential bands have loops forming two cycles per cell.
22. A stent according to claim 13, wherein the second
circumferential bands have loops forming three cycles per cell.
Description
FIELD OF THE INVENTION
The present invention relates generally to stents, which are
endoprostheses implanted into vessels within the body, such as
blood vessels, to support and hold open the vessels, or to secure
and support other endoprostheses in the vessels. In particular, the
present invention relates to a stent which is longitudinally
flexible before and after expansion.
BACKGROUND OF THE INVENTION
Various stents are known in the art. Typically stents are generally
tubular in shape, and are expandable from a relatively small,
unexpanded diameter to a larger, expanded diameter. For
implantation, the stent is typically mounted on the end of a
catheter, with the stent being held on the catheter at its
relatively small, unexpanded diameter. By the catheter, the
unexpanded stent is directed through the lumen to the intended
implantation site. Once the stent is at the intended implantation
site, it is expanded, typically either by an internal force, for
example by inflating a balloon on the inside of the stent, or by
allowing the stent to self-expand, for example by removing a sleeve
from around a self-expanding stent, allowing the stent to expand
outwardly. In either case, the expanded stent resists the tendency
of the vessel to narrow, thereby maintaining the vessel's
patency.
U.S. Pat. No. 5,733,303 to Israel et al. ("'303"), which is
expressly incorporated by reference, shows a unique stent formed of
a tube having a patterned shape which has first and second meander
patterns having axes extending in first and second directions. The
second meander patterns are intertwined with the first meander
patterns to form flexible cells. Stents such as this one are very
flexible in their unexpanded state such that they can be tracked
easily down tortuous lumens. Upon expansion, these stents provide
excellent radial support, stability, and coverage of the vessel
wall. These stents are also conformable, in that they adapt to the
shape of the vessel wall during implantation.
One feature of stents with a cellular mesh design such as this one,
however, is that they have limited longitudinal flexibility after
expansion, which may be a disadvantage in particular applications.
This limited longitudinal flexibility may cause stress points at
the end of the stent and along the length of the stent.
Conventional mesh stents like that shown in U.S. Pat. No. 4,733,665
may simply lack longitudinal flexibility, which is illustrated by
FIG. 1, a schematic diagram of a conventional stent 202 in a curved
vessel 204.
To implant a stent, it maybe delivered to a desired site by a
balloon catheter when the stent is in an unexpanded state. The
balloon catheter is then inflated to expand the stent, affixing the
stent into place. Due to the high inflation pressures of the
balloon--up to 20 atm--the balloon causes the curved vessel 204 and
even a longitudinally flexible stent to straighten when it is
inflated. If the stent, because of the configuration of its mesh is
or becomes relatively rigid after expansion, then the stent remains
or tends to remain in the same or substantially the same shape
after deflation of the balloon. However, the artery attempts to
return to its natural curve (indicated by dashed lines) in FIG. 1
with reference to a conventional mesh stent. The mismatch between
the natural curve of the artery and the straightened section of the
artery with a stent may cause points of stress concentration 206 at
the ends of the stent and stress along the entire stent length. The
coronary vasculature can impose additional stress on stents because
the coronary vasculature moves relatively significant amounts with
each heartbeat. For illustration purposes, the difference between
the curve of the vessel and the straightened stent has been
exaggerated in FIG. 1.
U.S. Pat. No. 5,807,404 to Richter, which is expressly incorporated
by reference, shows another stent which is especially suited for
implantation into curved arterial portions or osteal regions. This
stent can include sections adjacent the end of the stent with
greater bending flexibility than the remaining axial length of the
stent. While this modification at the end of the stent alleviates
the stress at the end points, it does not eliminate the stress
along the entire length of the stent.
Various stents are known that retain longitudinal flexibility after
expansion. For example, U.S. Pat. Nos. 4,886,062 and 5,133,732 to
Wiktor ("the Wiktor '062 and '732 patents") show various stents
formed of wire wherein the wire is initially formed into a band of
zig-zags forming a serpentine pattern, and then the zig-zag band is
coiled into a helical stent. The stents are expanded by an internal
force, for example by inflating a balloon.
The coiled zig-zag stents that are illustrated in FIGS. 1 through 6
of the Wiktor '062 and '732 patents are longitudinally flexible
both in the expanded and unexpanded condition such that they can be
tracked easily down tortuous lumens and such that they conform
relatively closely to the compliance of the vessel after
deployment. While these stents are flexible, they also have
relatively unstable support after expansion. Furthermore, these
stents leave large portions of the vessel wall uncovered, allowing
tissue and plaque prolapse into the lumen of the vessel.
Thus, it is desired to have a stent which exhibits longitudinal
flexibility before expansion such that it can easily be tracked
down tortuous lumens and longitudinal flexibility after expansion
such that it can comply with the vessel's natural flexibility and
curvature while still providing continuous, stable coverage of a
vessel wall that will minimize tissue sag into the lumen.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide a stent that is
longitudinally flexible before expansion so that it can easily be
tracked down tortuous vessels and remains longitudinally flexible
after expansion such that it will substantially eliminate any
stress points by complying with the vessel's flexibility and
assuming the natural curve of the vessel.
Embodiments of the present invention also to provide a stent that
is longitudinally flexible after delivery such that it flexes
during the cycles of the heartbeat to reduce cyclic stress at the
ends of the stent and along the stent. In some embodiments, the
stress experienced during such flexes is below the elastic limit of
the material and thus, a very high number of flexes, without
fatigue is possible
In addition, embodiments of the present invention provide a stent
with a closed cell pattern such that it provides good coverage and
support to a vessel wall after expansion.
A stent according to the invention retains the longitudinal
flexibility associated with the '303 cellular stent in its
unexpanded state, and has increased longitudinal flexibility in the
expanded state. The stent does so without sacrificing
scaffolding--i.e. coverage of the vessel wall--or radial
support.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic diagram of a conventional rigid stent
deployed in a curved lumen;
FIG. 2 shows a schematic diagram of a stent of the present
invention deployed in a curved lumen;
FIG. 3 shows a pattern for a stent made in accordance with the
present invention;
FIG. 4 shows an enlarged view of one cell of the pattern of FIG.
3;
FIG. 5 shows a pattern for a stent made in accordance with the
present invention;
FIG. 6 shows an enlarged view of one cell of the pattern of FIG.
5;
FIG. 7 shows a pattern for a stent constructed according to the
principles of the invention which has variable geometry along its
length.
FIG. 8A shows the expansion of a portion of a horizontal meander
pattern built according to the principles of the invention.
FIG. 8B shows the expansion of a portion of another horizontal
meander pattern.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 2 shows a schematic diagram of a longitudinally flexible stent
208 of the present invention. The stent 208 may be delivered to a
curved vessel 210 by a balloon catheter, and implanted in the
artery by inflating the balloon. As described before, the balloon
causes the artery to straighten upon inflation of the balloon.
However, upon deflation of the balloon, the stent 208 assumes the
natural curve of the vessel 210 because it is and remains
longitudinally flexible after expansion. This reduces any potential
stress points at the ends of the stent and along the length of the
stent. Furthermore, because the stent is longitudinally flexible
after expansion, the stent will flex longitudinally with the vessel
during the cycles caused by a heartbeat. This also reduces any
cyclic stress at the ends of the stent and along the length of the
stent.
FIG. 3 shows a pattern of a stent according to the present
invention. This pattern may be constructed of known materials, and
for example stainless steel, but it is particularly suitable to be
constructed from NiTi. The pattern can be formed by etching a flat
sheet of NiTi into the pattern shown. The flat sheet is formed into
a stent by rolling the etched sheet into a tubular shape, and
welding the edges of the sheet together to form a tubular stent.
The details of this method of forming the stent, which has certain
advantages, are disclosed in U.S. Pat. Nos. 5,836,964 and
5,997,973, which are hereby expressly incorporated by reference.
Other methods known to those of skill in the art such as laser
cutting a tube or etching a tube may also be used to construct a
stent which uses the present invention. After formation into a
tubular shape, a NiTi stent is heat treated, as known by those
skilled in the art, to take advantage of the shape memory
characteristics of NiTi and its superelasticity.
The pattern 300 is formed from a plurality of each of two
orthogonal meander patterns which patterns are intertwined with
each other. The term "meander pattern" is taken herein to describe
a periodic pattern about a center line and "orthogonal meander
patterns" are patterns whose center lines are orthogonal to each
other.
A meander pattern 301 is a vertical sinusoid having a vertical
center line 302. It will be recognized that this is not a perfect
sinusoid, but only an approximation thereof. Thus, as used herein,
the term sinusoid refers to a periodic pattern which varies
positively and negatively symmetrically about an axis; it need not
be an exact sine function. A meander pattern 301 has two loops 304
and 306 per period wherein loops 304 open to the right while loops
306 open to the left. Loops 304 and 306 share common members 308
and 310, where member 308 joins one loop 304 to its following loop
306 and member 308 joins one loop 306 to its following loop 304.
The vertical sinusoid of meander pattern 301 has a first
frequency.
A meander pattern 312 (two of which have been shaded for reference)
is a horizontal pattern having a horizontal center line 314. A
horizontal meander pattern 312 also has loops labeled 316, 318,
320, 322, and between the loops of a period is a section labeled
324. Looked at another way, these loops are part of a vertical
sinusoid 303 which has a higher frequency than that of the meander
patterns 301. Vertical sinusoids 301 alternate with vertical
sinusoids 303. Vertical sinusoids 303 have a second frequency
higher than the first frequency of the vertical meander patterns,
i.e., sinusoids 301.
Vertical meander pattern 301 is provided in odd and even (o and e)
versions which are 180N out of phase with each other. Thus, each
left opening loop 306 of meander pattern 301o faces a right opening
loop 304 of meander pattern 301e and a right opening loop 304 of
meander pattern 301o faces a left opening loop 306 of meander
pattern 301e.
The horizontal meander pattern 312 is also provided in odd and even
forms. The straight sections 324 of the horizontal meander pattern
312e intersect with every third common member 310 of the even
vertical meander pattern 301e. The straight sections 324 of the
horizontal meander pattern 312o also intersect with every third
common member 310 of the odd vertical meander pattern 301. Viewed
as vertical sinusoids 303, alternating sinusoids 303 are
intermittently coupled to the meander patterns 301. For example,
between points 315 and 317, where vertical pattern 303 is coupled
to vertical pattern 301e, there are two loops 306 and one loop 304
of vertical pattern 301e and three loops 319 and two loops 321 of
vertical pattern 303. This corresponds to two cycles of pattern
301e and 3 cycles of pattern 303. Similarly, between two points of
coupling between vertical pattern 301o and vertical pattern 303 are
two loops 304 and one loop 306, again making two cycles. There will
be three loops 321 and two loops 319, again equal to three cycles
of pattern 303.
Upon expansion of the stent, the loops of the vertical meander
patterns 301 open up in the vertical direction. This causes them to
shorten in the horizontal direction. The loops in the horizontal
meander pattern 312 open up both in the vertical direction and the
horizontal direction, compensating for the shortening of the loops
of the vertical meander patterns.
It should be noted that the loops of the horizontal meander pattern
312, which are the loops of the vertical pattern 303 in the present
invention avoids foreshortening in a self-expanding stent in a
particularly effective manner. A self-expanding stent formed of a
shape-memory alloy must be compressed from an expanded position to
a compressed position for delivery. As shown in FIG. 7, because of
the configuration of the loops 319 and 321 of the horizontal
meander pattern 312, when the stent is compressed from an expanded
position 602 to a compressed position 604, the length 606 of the
horizontal meander pattern (width of the vertical pattern 330)
naturally shrinks. Consequently, when the stent expands, the loops
319 and 321 elongate and compensate for the shortening of the
vertical meander patterns 301e and 301o as the vertical meander
patterns 301e and 301o expand. In contrast, a horizontal meander
pattern with such shapes as N-shapes will not naturally shrink
longitudinally when compressed from an expanded position 608 to a
compressed position 610, as illustrated in FIG. 8.
A stent formed from the pattern of FIG. 3 and made of NiTi is
particularly well suited for use in the carotid artery or other
lumens subject to an outside pressure. One reason is that because
the stent is formed of NiTi, it is reboundable, which is a
desirable property for stents placed in the carotid artery. The
other reason is that the stent of FIG. 3 offers excellent
scaffolding, which is particularly important in the carotid artery.
Scaffolding is especially important in the carotid artery because
dislodged particles in the artery may embolize and cause a
stroke.
FIG. 4 is an expanded view of one flexible cell 500 of the pattern
of FIG. 3. Each flexible cell 500 includes: a first member 501
having a first end 502 and a second end 503; a second member 504
having a first end 505 and a second end 506; a third member 507
having a first end 508 and a second end 509; and a fourth member
510 having a first end 511 and a second end 512. The first end 502
of the first member 501 is joined to the first end 505 of the
second member 504 by a first curved member 535 to form a first loop
550, the second end 506 of the second member 504 is joined to the
second end 509 of the third member 508 by a second curved member
536, and the first end 508 of the third member 507 is joined to the
first end 511 of the fourth member 510 by a third curved member 537
to form a second loop 531. The first loop 530 defines a first angle
543. The second loop 531 defines a second angle 544. Each cell 500
also includes a fifth member 513 having a first end 514 and a
second end 515; a sixth member 516 having a first end 517 and a
second end 518; a seventh member 519 having a first end 520 and a
second end 521; an eighth member 522 having a first end 523 and a
second end 524; a ninth member 525 having a first end 526 and a
second end 527; and a tenth member having a first end 529 and a
second end 530. The first end 514 of the fifth member 513 is joined
to the second end 503 of the first member 501 at second junction
point 542, the second end 515 of the fifth member 513 is joined to
the second end 518 of the sixth member by a curved member 539 to
form a third loop 532, the first end 517 of the sixth member 516 is
joined to the first end 520 of the seventh member 519 by a fifth
curved member 548, the second end 521 of the seventh member 519 is
joined to the second end 524 of the eighth member 522 at third
junction point 540 to form a fourth loop 533, the first end 523 of
the eighth member 522 is joined to the first end 526 of the ninth
member 525 by a sixth curved member 549, the second end 526 of the
ninth member 525 is joined to the second end 530 of the tenth
member 528 by a seventh curved member 541 to form a fifth loop 534,
and the first end 529 of the tenth member 528 is joined to the
second end 512 of the fourth member 510. The third loop 532 defines
a third angle 545. The fourth loop 533 defines a fourth angle 546.
The fifth loop 534 defines a fifth angle 547.
In the embodiment shown in FIG. 4, the first member 501, the third
member 507, the sixth member 516, the eighth member 522, and the
tenth member 528 have substantially the same angular orientation to
the longitudinal axis of the stent and the second member 504, the
fourth member 510, the fifth member 513, the seventh member 519,
and the ninth member 512 have substantially the same angular
orientation to the longitudinal axis of the stent. In the
embodiment shown in FIG. 4, the lengths of the first, second, third
and fourth members 501, 504, 507, 510 are substantially equal. The
lengths of the fifth, sixth, seventh, eighth, ninth and tenth
members 513, 516, 519, 522, 525, 528 are also substantially equal.
Other embodiments where lengths of individual members are tailored
for specific applications, materials of construction or methods of
delivery are also possible, and may be preferable for them. It can
be seen that each cell includes two cycles of the lower frequency
vertical pattern and three cycles of the higher frequency vertical
pattern. The first, second, third, and fourth members 501, 504,
507, 510 may have a width that is greater than the width of the
fifth, sixth, seventh, eighth, ninth, and tenth members 513, 516,
519, 522, 525, 528 in that cell. The differing widths of the first,
second, third, and fourth members and the fifth, sixth, seventh,
eighth, ninth, and tenth members with respect to each other
contribute to the overall flexibility and resistance to radial
compression of the cell. The widths of the various members can be
tailored for specific applications. For example, the ratio of width
may be approximately 50 70%. The fifth, sixth, seventh, eighth,
ninth, and tenth members may be optimized predominantly to enable
longitudinal flexibility, both before and after expansion, while
the first, second, third, and fourth members may be optimized
predominantly to enable sufficient resistance to radial compression
to hold a vessel open. Although specific members may be optimized
to predominantly enable a desired characteristic, all the portions
of the cell interactively cooperate and contribute to the
characteristics of the stent.
FIGS. 5 and 6 show a pattern and an expanded view of one cell of an
embodiment of the present invention which is specially adapted for
a stent made of stainless steel. The pattern is similar to the
pattern of FIGS. 3 and 4, and the same reference numerals are used
to indicate the generally corresponding parts.
The embodiments of FIGS. 3 and 5 can also be viewed as being made
up of high frequency and low frequency vertical sinusoidal patterns
or vertical loop containing sections which are arranged generally
in the circumferential direction and which are periodically
interconnected. Thus, there is a first loop containing section with
loops occurring at a first frequency extending along line 301 and a
second loop containing section with also occurring at said first
frequency extending along line 302. A third loop containing section
303 extending along line 305 has loops occurring at a second
frequency that is higher than said first frequency. It is disposed
between the first and second loop containing sections and
alternately joined to the first and second loop containing
sections. In the illustrated embodiment, the high frequency is in a
ratio of 3/2 to the low frequency. As noted above, the higher
frequency loop containing elements are smaller in width. The
relative widths can be selected so that the high frequency elements
are crimpable to the same diameter as the lower frequency elements.
The higher frequency elements provide improved flexibility.
Furthermore the high frequency vertical patterns of smaller width
result in elements having a lower maximal strain. Specifically,
when the stent is expanded, the lower maximal strain is below the
maximum strain without non-elastic deformation for the material of
the stent. In this embodiment where the stent is made of stainless
steel the lower maximal strain is below approximately 0.5%, even
for a 150 B bend, as confirmed by finite element analysis. On the
other hand, in a '303 type stent, for an equivalent amount of
bending, exhibits a maximum strain of 8%. Thus, the increased
flexibility of the stent of the present invention means that, in
addition to conforming better to the curved lumen, it will bend
with each beat of the heart. The strain during heartbeat happens
8,000,000 times every year and cannot be much above elastic limit
without the stent breaking. Since, embodiments of the present
invention keep the strain below the limit means that the stent of
the present invention can bend with the lumen as the heart beats,
for many years without breaking.
Also in this embodiment of the invention, for example, the second
loops 531 are made stronger by shortening the third and fourth
members 507, 510. This helps assure that the second loops do not
"flare out" during delivery of the stent through tortuous anatomy.
This "flaring out" is not a concern with NiTi stents which are
covered by a sheath during delivery.
Furthermore, the length of the members in this embodiment may be
shorter than the length of the corresponding members in the
embodiment illustrated in FIGS. 3 and 4. Typically, the amount of
strain allowed in a self-expanding NiTi stent may be around 10%. In
a stainless steel stent, the amount of strain allowed during the
plastic deformation which take place, for example, during
expansion, typically may be 20% or greater. Therefore, to
facilitate stents made of NiTi and stents made of stainless steel
expanding to comparable diameters, the members of the NiTi stent
may be longer than the members of a stainless steel stent.
In the particular embodiments described above, the stent is
substantially uniform over its entire length. However, other
applications where portions of the stent are adapted to provide
different characteristics are also possible. For example, as shown
in FIG. 7, a band of cells 850 may be designed to provide different
flexibility characteristics or different radial compression
characteristics than the remaining bands of cells by altering the
widths and lengths of the members making up that band. Or, the
stent may be adapted to provide increased access to a side branch
lumen by providing at least one cell 852 which is larger in size
then the remaining cells, or by providing an entire band of cells
854 which are larger in size than the other bands of cells. Note
that the cells 854 are formed by a first loop containing section
856, which arranged generally in the circumferential direction,
with the loops in first loop containing section 856 occurring at a
first frequency; a second loop containing section 858, which is
also arranged generally in the circumferential direction, with the
loops in the second loop containing section 858 also occurring at
the first frequency; and third loop containing sections 860, which
are arranged generally in the circumferential direction. The loops
in said third loop containing sections 860 occur at a second
frequency that is higher than said first frequency and are disposed
between and first and second loop containing sections and
alternately joined to said first and second loop containing
sections.
Or, the stent may be designed to expand to different diameters
along the length of the stent. The stent may also be treated after
formation of the stent by coating the stent with a medicine,
plating the stent with a protective material, plating the stent
with a radiopaque material, or covering the stent with a
material.
Thus, what is described is a longitudinally flexible stent that
utilizes a closed cell structure to provide excellent coverage of
the vessel wall. The general concepts described herein can be
utilized to form stents with different configurations than the
particular embodiments described herein. For example, the general
concepts can be used to form bifurcated stents. It will be
appreciated by persons skilled in the art that the present
invention is not limited to what has been particularly shown and
described above. Rather, the scope of the present invention is
defined by the claims which follow.
* * * * *